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Calculating Irrigation Demand: GPM and Total Dynamic Head (TDH)
Daily Water Demand Estimates Based on Type of Crop, Size of Field, and Local Evapotranspiration Data
The first step in proper irrigation planning is determining your daily water requirements. The formula you will use is:
GPM (Gallons Per Minute) = Total Water Requirement (TWR) / Irrigation Operating Time (IOT) in hours × 60
The Total Water Requirement (TWR) is determined by three main parameters: the type of crop, the size of the field, and the local Actual Evapotranspiration rate (AET). AET is a measure of how much water is lost into the atmosphere (including the water consumed by plants). For example, Corn requires approximately a quarter of an inch of water daily during its active growing phase. This translates to about 33,000 gallons of water daily for a five-acre field (since an acre inch is about 27,000 gallons). In this case, water application for a four hours daily would assume a flow rate of 140 GPM. Those relying on average estimates rather than obtaining AET data from USDA NRCS, county extension offices, etc., often end up overwatering or underwatering fields, resulting in crop stress and wastage of water resources.
To determine Total Dynamic Head: Static Lift, Pipe Friction Loss, and Required Discharge Pressure
Total Dynamic Head (TDH) defines the total energy required of a pump to move the water through your system, and consists of the following components:
Static Lift – the vertical distance (in feet) from the water source to the highest point of discharge
Friction Loss – the resistance attributed to the length, diameter, and material of the pipe as well as the flow rate, which can be calculated by industry-standard charts (Hazen-Williams) or other online resources, such as the PVC Pipe Friction Loss Calculator
Discharge Pressure – the minimum pressure (in PSI) that must be present at the emitters (i.e. evaporative sprinklers require 15- 60 PSI, and drip emitters require 10 - 30 PSI) pressure which can be converted to feet by using the equation PSI x 2.31
Total Dynamic Head (TDH) in feet can be calculated as: Static Lift + (Friction Loss in feet) + (Discharge Pressure in psi x 2.31). For example: a system with a static lift of 50 feet, 200 feet of 2-inch PVC pipe (with a friction loss of about 8 feet at a flow rate of 141 gallons per minute), and a discharge pressure of 20 psi, would give the following TDH = 50 + 8 + (20 x 2.31) = about 104 feet. Pumps require a lot of time and effort to mess with the TDH. When TDH remains underestimated, pumps are forced to work harder and ultimately wear out and breakdown much sooner. This can significantly shorten the life of the pump by as much as half the normal range of lifespans, as mentioned in the U.S. Department of Energy's guide to solar pumping systems.
Performance Curves and Application Matching for Optimal Selection of Demax Solar Water Pumps
Surface and Submersible Pumps: Choosing the Right Pump Based on Well Depth, Water Table, and Field Layout
It's not just a water source's depth that influences pump selection. Surface pumps are above-ground and best suited for shallow sources, such as ponds and streams, that are less than 20 feet deep. Their efficiency is enhanced when installed on flat terrain with minimal vertical obstacles. Submersible pumps are ideal for wells deeper than 20 feet, as they can pull water from beneath the water table. These pumps are especially useful in areas with seasonal fluctuations in groundwater levels. Additionally, terrain impacts pump selection. Surface pumps are less effective on slopes over 10%. In contrast, submersible pumps can be installed on rugged terrain, as they are in close proximity to the water source. Before installation, it is critical to measure the current and historic low points of the water table. Demax technicians found that approximately 66% of early pump failures could have been avoided with this basic understanding.
Understanding GPM–Head Curves for Matching Demax Solar Water Pump Output to Your Total Dynamic Head and Flow Requirements
Demax solar pumps flow curves show achievable flow (GPM) in relation to Total Dynamic Head (TDH). These curves, available for each Demax model, are essential for aligning your hardware to real-world demand. To do this accurately:
Mark your calculated TDH on the vertical axis
Move right to the performance curve
Read the GPM on the horizontal axis
When choosing a model, consider one where the curve shows performance above your requirement at given conditions, say approx 141 GPM at 104 FT TDH. Aim for about 10 to 15 percent more to consider real world issues on the installation like scaling in the pipes, dirt on the panels, reduction in electrical voltage, and so on without causing the pump to overheat. Avoid working conditions right at the upper right corner as this shows a low performing pump with significant motor and performance issues. The Demax performance charts consider different temp and sunlight conditions as well as addition real world adjustments which are more important than lab test data for accurate sizing.
Properly Sizing Your Solar System for Reliability and Efficiency
Photovoltaic array size must take into consideration three particular scenarios:
1. Starting surge, which can cause a multi-factor 2-3x running watts found more commonly in high-inertia submersible pump motors, and
2. Daily energy demand estimated by pump watts x daily run-time. For example, a 1.5 kW pump running for four hours requires 6 kWh/day.
3. Real-world losses, 15-25% due to panel heating, dust, wiring, and in AC systems, inverter inefficiencies.
Under-sizing solar arrays can leave certain pumps non-operational due to energy supply shortages during cloudy days or during the early hours of the day when the sun is out limitedly. On the contrary, excessive sizing increases operational costs for minimal increases in functional output. A useful strategy is to take daily utilization energy demand in kWh and multiply it by a factor of 1.25 to arrive at a conservative estimate for system losses. To complete the sizing, divide the result by the number of peak sun hours available at the location. For example, a pump that has a rating of 2 horsepower (roughly 1.5 kW) and a daily energy requirement of 6 kWh. With an assumption of 5 peak sun hours the simple mathematics show 1.5 x 1.25 / 5 = 0.375 kW is required. It is reasonable to assume 600 watts of panel capacity would be required. Always check equipment manufacturers guidelines as they may provide additional insight or guidance into sizing.
Demax spec numbers show that a minimum of 1.3 times the DC input wattage is needed to keep things running under full load conditions.
DC vs. AC vs. Hybrid Systems: Which Solar Water Pump System Configuration is Best?
Determining system architecture should be very specific to the desired reliability, accessible infrastructure, and climate patterns.
System Type | Best For | Key Advantages
DC | Remote, off-grid small- to mid-scale irrigation | Highest overall efficiency (no inverter losses); simple installation
AC | Grid-connected farms needing backup or shared infrastructure | Seamless integration with existing electrical systems; easier scalability
with variable-speed drives
Hybrid | Regions with frequent cloud cover or monsoonal variability | Battery buffer ensures constant operation during low-irradiance periods—critical for sensitive crops
When every single watt is vital for the farm, DC systems are the call for standalone applications. If the farm has existing grid connections, they should opt for AC as it is a better choice for future hybrid integration. Hybrid systems come at a higher initial cost, but for farmers with inflexible watering schedules, these systems are the most valuable. For instance, frost protection of orchards at night and maintaining the moisture of high-value crops is an important operational need. Furthermore, during extended periods of cloud cover, farms using hybrid systems for irrigation lost just 28% of crops, compared to those only using DC systems, showing off the value of these systems. This information from UC Davis, published in 2023, is the kind of difference that adds up quickly.
Make the Right Choice for the Highest ROI and the Longest System Life
Choosing the wrong solar water pump means the money spent will be lost, and not only because of failures, inefficient operation, and lifecycle costs that are hard to define. Some examples include:
Choosing the lowest initial cost without considering total cost of ownership (TCO) due to costs associated with energy efficiency, frequency of maintenance, extended warranty, operational lifespan, etc.
Selecting pumps that are not environmentally appropriate. For example, many pumps rated only for 25°C ambient will fail sooner than expected when used in desert or tropical climate conditions without without thermal derating or an IP68 rated housing.
Neglecting the necessary compliance and water quality. For example, high iron content or saline water will quickly corrode standard cast iron impeller and housing and will demand the use of stainless steel or other specialty impeller materials.
There are circumstances in the field that improve specs. Take pumps for example. A model could claim 150 gallons per minute at 100 feet total dynamic head… But solar panels run hot at 65 degrees Celsius. It could also be that the inlet filter is stagnated with biological growth. Demax has developed their own field testing approach that works across thousands of installations worldwide. The process involves matching equipment performance data with site specific factors like local sunlight patterns, water composition analysis, and adjusted pressure needs based on elevation changes. When installers skip these checks, they end up with systems that are either too small leading to constant watering issues or way too big which causes problems like cavitation damage and premature bearing wear. Industry studies show this oversight leads to sizing errors affecting more than half of all installations.
Frequently Asked Questions (FAQ)
What is Total Dynamic Head (TDH)?
TDH measures total energy for a pump to move water through a system. It is made up of static lift, pipe friction loss, and discharge pressure.
Why is precise calculation of irrigation demand critical?
This prevents resource wastage through over-irrigation.
What factors should be taken into account when choosing a solar water pump?
Consider the pump’s environmental compatibility, total cost of ownership, and any applicable regulations.
What is the function of solar pump performance curves?
They show flow vs TDH which allows you to select the appropriate pump for your use case.